The conventional wood pulping processes widely used in the pulp and paper industry produce very large quantities of a waste material known as kraft black liquor. Direct discharge of the kraft black liquor into the environment outside the pulp and paper plant is not commercially practicable, because it would cause very severe pollution problems. Furthermore, the kraft black liquor contains useful pulping chemicals and is also a potential source of process heat needed in the wood pulping process. Accordingly, it is conventional practice in the pulp and paper industry to concentrate the kraft black liquor and then to burn the resultant concentrate, with recovery of useful heat and concurrent recovery of valuable pulping chemicals. Although this concentration/burning process is greatly preferable to direct discharge of the kraft black liquor into the environment, it does have the disadvantage that the kraft black liquor contains a substantial amount of low molecular weight organic acids and their derivatives which have a low heat value when burned but which, if they could be separated from the black liquor, would have substantially greater value for other purposes, for example as fermentation feedstocks and chemical intermediates. While many studies of separation and purification of the various constituents of kraft black liquor have hitherto been carried out in an attempt to recover useful lignin from the liquor, relatively little attention has been directed to processes capable of economically recovering in a reasonably pure and concentrated form the organic acid constituents of the black liquor.
U.S. Pat. No. 1,271,591 issued July 9, 1918 to Loomis describes the formation of a number of fatty acids from wood and plant material by heating or by dissolution and extraction of resinous and gummy materials with fatty acids. This process is totally inapplicable to kraft black liquor.
U.S. Pat. No. 1,723,880 issued Aug. 6, 1929 to Michael et al. describes a process for preparing ammonium salts of organic acids from sulphite cellulose waste liquors by a process in which the waste liquor is heated at high pressure and the carbonaceous material produced separated, thereby producing a solution containing the ammonium salts of the organic acids produced by decomposition of the lignin in the wood. If desired, the calcium salts of these organic acids can be produced by adding calcium carbonate to the treated liquor. Again, the process is inapplicable to the treatment of kraft black liquor.
U.S. Pat. No. 1,859,888, issued May 24, 1932 to Richter describes a process for the regeneration of spent alkaline liquor resulting from the digestion of cellulosic material. In the regeneration process, the spent liquor is treated with sulphuric acid, which reacts with the sodium salts of organic acids present in the spent liquor, forming sodium sulphate and resulting in the coagulation of organic matter which can thus be separated from the remainder of the spent liquor. However, the process does not result in the separation of a low molecular weight aliphatic organic acid fraction in a reasonably pure form.
U.S. Pat. No. 2,701,255 issued Feb. 1, 1955 to Heritage et al. describes a complicated multi-stage process for separating organic products from lignocellulose. The first step of the multi-stage process is acidification to approximately pH 1.5, followed by filtration to remove the lignin precipitated by the acidification. The later stages of the process involve treatment of the delignified solution with alkaline earth metal compounds, lead compounds and alkali metal carbonates, followed by steam distillation to produce a volatile organic acid fraction. The process is not intended for treatment of kraft black liquor, and in view of its complexity it seems doubtful whether it is capable of being economically operated to produce a volatile organic acid fraction.
Finally, U.S. Pat. No. 2,926,114 issued Feb. 3, 1960 to Grangaard et al., describes a process for producing wood pulp and an organic fraction from wood by treating wood fragments in an aqueous solution of pH 7-9 at a temperature of 120-160.degree. C. under strongly oxygenating conditions. Again, the process is not intended for the treatment of kraft black liquor.
Accordingly, it will be seen that there is a need for a process which can economically recover the low molecular weight aliphatic acids present in kraft black liquor in a reasonably pure form, while still leaving the remaining components of the liquor available for the usual combustion process, and this invention seeks to provide such a process.